Since their introduction in the 1950's screw compressors have steadily become more important as sources of compressed air and gas. Essentially a screw compressor incorporates an air end having a housing with inlet and delivery ports and containing a pair of rotors having intermeshing helical lobes. Rotation of the rotors induces gas provided at the inlet port which is subsequently compressed in an axially moving progressively reducing helical space and delivered at the delivery port. A critical factor in the efficiency of screw compressors is the rotor clearance gaps between the rotors which although necessary as a running clearance does allow gas leakage. Very approximately a reduction of 0.01 mm in the clearance increases efficiency by one per cent.
From this it can be seen that very accurate manufacture is required but methods of inspecting and checking the clearance gap have not kept pace with the demands placed upon them. In fact the only shop floor method available for measuring the clearance gap typically in the range zero to 100 microns is by use of feeler gauges which can enter the curved clearance spaces and detect the points of minimum clearance.
However, the use of feeler gauges is very dependent on the human factor and it is generally felt that the method is insufficiently accurate for modern requirements.
It is therefore an object of the present invention to provide an improved method of measuring small gaps in general and the rotor clearance gaps of screw compressors in particular.
The present invention is a method of measuring a gap comprising directing a beam of light towards the gap, measuring the amount of paraxial light passing through the gap and determining the width of the gap from the measured amount of paraxial light.
The present invention is also a method of measuring the rotor clearance gap between the rotors of a screw compressor comprising directing a beam of light parallel to the pitch line helix angle towards the gap, measuring the amount of paraxial light passing through the gap and determining the width of the gap from the measured amount of paraxial light.
The present invention is further apparatus for measuring a gap comprising a light projector for generating a beam of light, a detector system including a detector for measuring the amount of paraxial light falling thereon and means for determining from the measured amount of paraxial light the width of a gap located between the projector and the detector system, the detector system including means for focusing an image of the gap on the detector.